Reaction of Metals with Bases: Sodium Zincate | Class 10

Quick answer: Most metals do not react with bases, but some, such as zinc and aluminium, do. When zinc is heated with sodium hydroxide solution, it forms sodium zincate and hydrogen gas: $\mathrm{Zn + 2NaOH \rightarrow Na_2ZnO_2 + H_2}$. The hydrogen can be identified because it burns with a pop sound. Aluminium reacts in a similar way to form sodium aluminate and hydrogen.

Metals usually react with acids to give a salt and hydrogen gas. You might expect them to react with bases in the same way, but most metals, such as iron and copper, do not.

A few metals are exceptions. Zinc and aluminium react with a strong base such as sodium hydroxide and give off hydrogen gas.

In CBSE Class 10 Science, Chapter 2 (Acids, Bases and Salts), the reaction of zinc with sodium hydroxide is an NCERT activity, and it links to the idea of amphoteric oxides in Chapter 3 (Metals and Non-metals).

What You Will Learn

• Why most metals do not react with bases
• The reaction of zinc with sodium hydroxide, forming sodium zincate
• The reaction of aluminium with sodium hydroxide, forming sodium aluminate
• How to test for hydrogen gas
• What amphoteric oxides are

1. Metals and Bases

Key point: Only some metals, such as zinc and aluminium, react with bases to give hydrogen. Not all metals do.

2. Zinc and Sodium Hydroxide

Step 1: Take a few pieces of granulated zinc in a test tube and add sodium hydroxide solution.

Step 2: Warm the test tube. Bubbles of gas are given off.

Equation:

$\mathrm{Zn(s) + 2NaOH(aq) \xrightarrow{heat} Na_2ZnO_2(aq) + H_2(g)}$

Why $\mathrm{Na_2ZnO_2}$? The zincate ion is $\mathrm{ZnO_2^{2-}}$, which has a charge of $2-$, so two sodium ions ($\mathrm{Na^+}$) are needed.

3. Aluminium and Sodium Hydroxide

Aluminium reacts with sodium hydroxide solution (in the presence of water) to form sodium aluminate and hydrogen:

$\mathrm{2Al(s) + 2NaOH(aq) + 2H_2O(l) \rightarrow 2NaAlO_2(aq) + 3H_2(g)}$

4. Testing for Hydrogen Gas

Step 1: Pass the gas through a delivery tube into soap solution, so that bubbles filled with the gas form.

Step 2: Bring a burning candle or matchstick near a gas-filled bubble.

Step 3: The gas burns with a pop sound. This shows that the gas is hydrogen.

5. Amphoteric Oxides

Definition: Amphoteric oxides are metal oxides that react with both acids and bases to form salt and water. Zinc oxide and aluminium oxide are examples.

$\mathrm{Al_2O_3 + 6HCl \rightarrow 2AlCl_3 + 3H_2O} \quad \text{(behaves as a base)}$

$\mathrm{Al_2O_3 + 2NaOH \rightarrow 2NaAlO_2 + H_2O} \quad \text{(behaves as an acid)}$

The reaction of aluminium oxide with sodium hydroxide is used in industry to purify bauxite, the ore of aluminium.

6. Summary Table

7. Exam Tips and Common Mistakes

Exam Tip: Name the product correctly: zinc and sodium hydroxide give sodium zincate, $\mathrm{Na_2ZnO_2}$.

Exam Tip: For the hydrogen test, use a burning splint or candle, and mention the pop sound.

Common Mistake: Writing sodium zincate as $\mathrm{NaZnO_2}$. Because the zincate ion has a $2-$ charge, the formula is $\mathrm{Na_2ZnO_2}$.

Common Mistake: Saying all metals react with bases. Only some metals, like zinc and aluminium, do.

Frequently Asked Questions

What happens when zinc reacts with sodium hydroxide?

Zinc reacts with sodium hydroxide on heating to form sodium zincate and hydrogen gas: Zn + 2NaOH → Na₂ZnO₂ + H₂.

What is the formula of sodium zincate?

Na₂ZnO₂.

Do all metals react with bases?

No. Most metals, such as iron and copper, do not. Zinc and aluminium are examples of metals that do.

How do you test for hydrogen gas?

Bring a burning matchstick or candle near the gas; it burns with a pop sound.

What is an amphoteric oxide?

A metal oxide that reacts with both acids and bases to form salt and water, such as zinc oxide and aluminium oxide.